This invention relates to a ceramic, and more i particularly, but not exclusively, to a sintered body of silicon nitride, with high density, toughness, and strength and also to its manufacture. This ceramic is appropriate for use in automobile engine parts (e.g., a ceramic rotor or a ceramic valve) or for use as wear parts (e.g., ball bearing).
Silicon nitride (Si.sub.3 N.sub.4) ceramics have been receiving attention for use as structural or wear parts in apparatus that are operated at high temperatures (e.g., a high temperature gas turbine, a diesel engine, and an MHD generator) since they possess fairly high strength even at high temperatures, high chemical stability, and high thermal shock resistance.
Silicon nitride ceramics are dense sintered bodies generally prepared by the additions (e.g., Y.sub.203, Al.sub.2 O.sub.3, or MgO) to powdery Si.sub.3 N.sub.4 and then sintering the mixture because it is difficult to sinter powdery Si.sub.3 N.sub.4 by itself. When Y.sub.2 O.sub.3 or MgO is used as sintering additive, a sintered body is obtained in which Si.sub.3 N.sub.4 particles are columnar crystals with high aspect ratios. A dense sintered body consisting of columnar crystals twined together, generally has a fairly high degree of strength and toughness. One example of such silicon nitride ceramics is disclosed in Japanese Published Unexamined Patent Application No.S60-137873.
General silicon nitride ceramics, however, have a problem: they do not possess sufficient strength and toughness to be used as structural parts, and their hardness could also be improved.